Process for the electrodeposition of a colored nickel-tin alloy coating

ABSTRACT

ELECTROPLATING APPARATUS FOR PLATING AN ARTICLE IMMERSED IN AN ELECTROLYTIC BATH BY APPLYING A POSITIVE DC VOLTAGE TO AN ANODE IMMERSED IN THE ELECTRLYTIC BATH AND BY APPLYING A NEGIVE DC VOLTAGE TO THE ARTICLE, COMPRISES AN ELECTROLYTIC BATH CONTAINING AN AQUEOUS SOLUTION OF NICKEL IN A RANGE OF 5 TO 100 GGRAMS LITER AND PROVIDED BY ONE OF A GROUP CONSISTING OF NICKEL SULFATE, NICKEL SULFAMATE, NICKEL FLUOBORATE, NICKEEL CHLORIDE, NICKEL CARBONATE, NICKEL CYANIDE, NICKEL ACETATE, NICKEL ETHYLENE DIAMINE TETRA ACETIC ACID, NICKEL OXIDE AND NICKEL TARTATE, TIN IN A RANGE OF 5 TO 100 GRAMS PER LITER AND PROVIDED BY ONE OF A GROUP CONSISTING OF SODIUM STANATE, POTAS-   SIUM STANATE, STANNIC CHLORIDE, STANNOUS CHLORIDE, STANNIC SULFATE, STANNOUS SULFATE, STANNOIC FLUOBORATE AND STANNOUS FLUOBORATE, ALKALI CYANIDE IN A RANGE OF 5 TO 200 GRAMS PER LITER PROVIDED BY ONE OF A GROUP CONSISTIN OF SO DIUM CYANIDE AND POTASSIUM CYANIDE, AND AT LEAST ONE OF A GROUP CONSISTING OF COPPER, ZINC MOLYBDENUM, SELENIUM ARSENIC AN SULFUR IN A RANGE OF 0 TO 20 GRAMS PER LITER.

L. H. CATES ET AL FROCESS FOR THE ELECTRODEPOSITLON OI" A COLOREDNICKEL-TIN ALLOY COATING Original Filed Oct. 6, 1967 VOLTA/ffm? 22 D OTHEIR ATTORNEY United States Patent O Int. Cl. C23f 5/38 U.S. Cl. 204-431 Claim ABSTRACT F THE DISCLOSURE Electroplating apparatus for platingan article irnmersed in an electrolytic bath by applying a positive DCvoltage to an anode immersed in the electrolytic bath and by applying anegative DC voltage to the article, comprises an electrolytic bathcontaining an aqueous solution of nickel in a range of to 100 grams perliter and provided by one of a group consisting of nickel sulfate,nickel sulfamate, nickel uoborate, nickel chloride, nickel carbonate,nickel cyanide, nickel acetate, nickel ethylene diamine tetra aceticacid, nickel oxide and nickel tartrate, tin in a range of 5 to 100 gramsper liter and provided by one of a group consisting of sodium stannate,potassium stannate, stannic chloride, stannous chloride, stannicsulfate, stannous sulfate, stannic uoborate and stannous tluoborate,alkali cyanide in a range of 5 to 200 grams per liter provided by one ofa group consisting of sodium cyanide and potassium cyanide, and at leastone of a group consisting of copper, zinc, molybdenum, selenium, arsenicand sulfur in a range of 0 to 2O grams per liter.

BACKGROUND OF THE INVENTION (A) Field of the invention This is acontinuation of application Ser. No. 621,002, tiled Oct. 30, 1967, nowabandoned, and relates to an alloy cyanide electroplating process. Moreparticularly, the invention relates to an alkaline alloy cyanideelectroplating process which produces a variety of colored deposits.

(B) Description of the prior art The electroplating of metals is a wellknown art, as evidenced by the McGraw-Hill Encyclopedia of Science andTechnology, McGraw-Hill Book Company, Inc., vol. 4, 1960, pp. 528 to535; Principles of Electroplating and Electroforming, by W. Blum and G.B. Hogaboom, third edition, 1949; Protective Coatings for Metals, by R.M. Burns and W. W. Bradley, third edition, A.C.S. Monograph 129, 1959;Electroplating Engineering Handbook, A. K. Graham, editor, 1955; ModernElectroplating, A. G. Gray, editor, 1953.

In electroplating, the cleaned article to be plated is connected as thecathode in a solution known as the electrolyte. Direct current isintroduced through the anode, which usually consists of the metal to bedeposited. Metal dissolves from the anode and deposits on the cathode.Under ideal conditions, the same weight of metal dissolves from theanode as is deposited on the cathode, and the overall composition of thebath remains constant. These conditions are never fully realized, andthe bath composition changes and must be adjusted at intervals. If theanode eiciency exceeds the cathode eiliciency, the metal content of thesolution increases and the pH of the solution increases, and vice versa.

The constituents of a plating bath include a soluble compound orcompounds of the metal or metals to be deposited, together with othersubstances added in fairly ice large amounts to increase electricalconductivity, throwing power, or some other property. Smallconcentrations of certain addition agents or brighteners, are employedto yield smoother or brighter deposits. Acid plating baths are cheaperto prepare and maintain, but the alkaline baths, which consistprincipally of complex cyanides, have better throwing power and yieldfiner-grained deposits.

The throwing power represents the ability of a solution to producecoatings of uniform thickness on surfaces where the distances betweenvarious portions of the surface andthe anode differ.

The weight of metal deposited depends on the quantity of electricity, incoulombs or ampere hours, that is passed to the cathode. The averagethickness of the coating depends upon the current density, expressed inamperes per square decimeter or per square foot, and the period ofdeposition. The uniformity of the coating thickness depends upon theshape of the of the article, its position with respect to the anodes andthe throwing power of the solution.

The current density is an important factor in plating. To produce agiven current density, it is necessary to apply a suitable potential,expressed in volts. The total bath potential includes: the decompositionpotential; the IR drop, which depends upon the resistivity of the bathand the distance between the anodes and cathodes; the cathodepolarization; and the anode polarization at the prevailing currentdensities.

Nickel is extensively applied in electroplating. It is more resistant toatmospheric corrosion than other metals except nobel metals such asgold, and is fairly hard and wear-resistant. Nickel is passive in air,and hence if nickel coatings could be made impervious initially theyshould remain so in service.

Most nickel plating is done in baths containing nickel sulfate, nickelchloride and boric acid. Organic and inorganic additions are made toproduce bright deposits. By control of the bath composition, temperatureand current density, it is possible to vary widely the physicalproperties of the nickel deposits.

To deposit adherent coatings it is necessary to have the surface of thebasis metal clean, that is, free from all foreign substances such asgrease, and compounds such as oxides or suldes. The two essential stepsare cleaning and pickling.

Most plating operations are conducted with direct current at potentialsof 6 to 12 volts. The plating tanks are usually made of steel, whichrequires no lining for alkaline solutions.

Small objects are plated in barrels, usually hexagonal prisms, withperforated plastic sides, that rotate on a horizontal axis in a tankcontaining the plating solution and anodes. The articles contactcathodic connections as they tumble during rotation of the barrel.

SUMMARY OF THE INVENTION The principal object of the present inventionis to provide a new and improved colored alloy cyanide electroplatingprocess. The electroplating process of the present invention provides anadherent coating having good corrosion resistance, excellent shelf-lifeand good wear and abrasion characteristics. The electroplating processof the present invention provides a deposit which may be black,blue-black, blue, brown or grey nish on all basis materials which acceptan electroplate. The deposit provided by the electroplating process ofthe present invention is in any of a selected variety of nishes andretains the character of the surface plated. The deposit provided by theelectroplating process of the present invention is uniform, adherent andeasily controlled, and is produced in a single step. The electroplatingprocess of the present invention plates an additional coating of metal,rather than removing metal, so that it provides an appreciable saving inthe coloring of silver. The electroplating process ofthe presentinvention is applicable to barrel plating.

In accordance with the present invention, in electroplating apparatusfor plating an article immersed in an electrolytic bath by applying apositive DC voltage to an anode immersed in the electrolytic bath and byapplying a negative DC voltage to the article, an electrolytic bathcomprises an aqueous alkaline cyanide solution of nickel in a range of 5to 100 grams per liter, tin in a range of 5 to 100 grams per liter andalkali cyanide in a range of 5 to 200 grams per liter. The electrolyticbath further comprises at least one of a group comprising copper, zinc,molybdenum, selenium, arsenic and sulfur in a range of to 20 grams perliter. The nickel is provided by one of a group comprising nickelsulfate, nickel sulfamate, nickel fluoborate, nickel chloride, nickelcarbonate, nickel cyanide, nickel acetate, nickel ethylene diamine tetraacetic acid, nickel oxide and nickel tartrate. The tin is provided byone of a group comprising sodium stannate, potassium stannate, stannicchloride, stannous chloride, stannic sulfate, stannous sulfate, stanniciluoborate and stannous uoborate. The alkali cyanide is provided by oneof a group comprising sodium cyanide and potassium cyanide. The zinc,molybdenum, selenium, and etc., is provided from any of the watersoluble salts of these elements.

In accordance with the present invention, an electroplating process forplating an article comprises immersing the article in an electrolyticbath comprising an aqueous alkaline cyanide solution of theaforedescribed composition. A positive DC voltage is applied to an anodeimmersed in the electrolytic bath. A negative DC voltage is applied tothe article. The electrolytic bath is maintained at a determinedtemperature and a determined current density is provided for adetermined period of time. The electrolytic bath is maintained at atemperature in a range of 110 to 190 P. and preferably in a range of 140to 170 F. The current density is in a range of 2 to 250 amperes persquare foot and preferably in a range of 5 to 100 amperes per squarefoot.

BRIEF DESCRIPTION OF 'IHE DRAWING In order that the present inventionmay be readily carried into effect, it will now be described withreference to the accompanying drawing, wherein the single figure is atop View, partly in section, of electroplating apparatus which mayaccomplish the electroplating process of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Any suitable electroplatingapparatus known in the art may be utilized to accomplish theelectroplating process of the present invention. Suitable electroplatingapparatus may comprise, for example, that shown in the gure. The tank 11may comprise steel, stainless steel or rubber-lined steel and houses theelectrolyte or electrolytic bath 12, the anodes 13A and 13B immersed inthe electrolyte and the cathodes 14 immersed in the electrolytesubstantially half way between the anodes 13A and 13B.

The anodes 13A and 13B may be one or more in number and the cathodes 1'4may be one or more in number. The anodes 13A and 13B may comprise steel,stainless steel, graphite, nickel or other suitable electricallyconductive material, or the tank 11 may be made anodic.

A positive potential is applied to the anodes 13A via a lead 15Aconnected from the positive polarity terminal of a source 16 of DCvoltage of suitable magnitude such as, for example, 6 to 12 volts, and apositive potential is applied to the anodes 13B via a lead 15B connectedfrom said positive polarity terminal via the lead 15A. The DC voltagesource 16 may comprise any suitable source of DC voltage such as, forexample, a battery or transformer, rectifier circuit connected to an ACpower supply line. A negative potential is applied from the DC source 16to the cathodes 14 via a lead 17, a variable resistor or rheostat 18, anammeter 19 and a lead 21. A voltmeter 22. is connected across the seriescircuit arrangement of the DC voltage source 16, the variable resistor18 and the ammeter 19.

In accordance with the present invention, the electrolyte orelectrolytic bath 12 is an aqueous alkaline cyanide solution comprising5 to 100 grams per liter of nickel, 5 to 100 grams per liter of tin and5 to 200 grams per liter of alkali cyanide. Other materials such as, forexample, copper, zinc, molybdenum, selenium, arsenic or sulfur, may beadded to the solution in amounts ranging from 0 to 20 grams per liter.Although the additional materials do not effect the colorcharacteristics of the produced plated coating, they may modify therange, tone and uniformity of said coating.

The nickel is provided by any suitable nickel cornpound such as, forexample, nickel sulfate, nickel sulfamate, nickel iduoborate, nickelchloride, nickel carbonate, nickel cyanide, nickel acetate, nickelethylene diamine tetra acetic acid, nickel oxide, nickel tartrate, andso forth. The tin is provided by any suitable tin compound such as, forexample, sodium stannate, potassium stannate, stannic chloride, stannouschloride, stannic sulfate, stannous sulfate, stannic uoborate, stannousuoborate, and so forth. The alkali cyanide is provided by any suitablealkali cyanide compound such as, for example, sodium cyanide, potassiumcyanide, and so forth.

The electrical conductivity of the electrolytic bath 12 may be increasedby the addition of carbonates, sulfates, chlorides, tartrates, acetates,borates, and so on. These materials are not essential to the basicoperation of the process. Surface active materials, not essential to thebasic operation of the process, may be added to the electrolytic bath toaid in rinsability.

The electrolytic bath 12 is maintained at a temperature in the range ofto 190 F. and a current density in the range of 2 to 250 amperes persquare foot. The preferred temperature range is to 170 F. and thepreferred current density range is 5 to 100 amperes per square foot. Theelectrolytic bath 12 is heated by any suitable heating means such as,for example, steel heating coils, stainless steel thermopanels, steelimmersion heaters or direct gas ltired heating apparatus. Theelectrolytic bath 12 may thus be heated, for example, by immersionheaters 23A and 23B.

When the electroplating apparatus is 'operated at an electrolytic bathtemperature of from 140 to 165 F. and a current density of from 5 to 60amperes per square foot, a uniform grey or black coating is produced onany electrically conductive material connected as the cathode within 45seconds. If the plating time is increased, thevthickness of the coatingis increased accordingly. The plating time is preferably in the range of45 to 120 seconds.

If the electroplating process of the present invention is undertaken ata slow plating speed, or low temperature and low current density for ashort plating time, a brownish colored coating is produced. The initialbrownish color is darkened to fblue or blue-black when the plating timeis lengthened.

Brownish or bluish-black coatings are produced by varying thecomposition of the electrolytic bath 12. Brownish and bluish coatingsare produced at lower concentrations of metal and/ or higherconcentrations of alkali cyanide.

The color of the coating is also influenced by the brightness of thesubstrate or article to be plated. If the substrate is dull, the coatingis grey or greyish-black. If the substrate is bright, the coatingappears black. The limits `of the composition of the electrolytic bathare broad and the coating is a uniform grey, grey-black or black overwide temperature and current density ranges.

The electrolyte may be prepared as a solid composition of the properchemical formulation and the electrolytic bath 12 may be prepared fromthe solid composition by adding water in a proper amount to dissolvesaid solid composition. The electrolyte may also be prepared as aconcentrated liquid solution of the proper chemical formulation and theelectrolytic bath 12 may be prepared from the concentrated liquidsolution fby adding waterwin a proper amount. The electrolytic bath 12may, ofcourse, be prepared by adding the proper chemicals in the properconcentrations to water.

While the invention has been described by means of a specific exampleand in a specic embodiment, we do not wish to be limited thereto, forobvious modifications will occur to those skilled in the art withoutdeparting from the spirit and scope of the invention. l

What we claim is:

1. A process for electrodepositing a selected black, blueblack, blue,brown or grey nish on a metal article, comprising the steps of immersingthe article in an electrolytic bath comprising an aqueous alkalinecyanide solution of nickel of a group consisting of nickel sulfate,nickel sulfamate, nickel uoborate, nickel chloride, nickel carbonate,nickel cyanide, nickel acetate, nickel ethylene diamine tetra aceticacid, nickel oxide and nickel tartrate in a range of 5 to 100 grams perliters, tin of a a 6 group consisting of sodium stannate, potassiumstannate, stannic chloride, stannous chloride, stannic sulfate, stannoussulfate, stannic fluoborate and stannous flulborate in a range of 5 to100 grams per liter and alkali cyanide of a group consisting of sodiumcyanide and potassium cyanide in a range of 5 to 200 grams per liter;applying a positive DC voltage to an anode immersed in the electrolyticbath; applying a negative DC voltage to a metal article; and maintainingthe electrolytic bath at a determined temperature in a range of to 170F. and providing a determined current density in a range of 5 to 100amperes per square foot for a determined period of time.

References Cited Electrodeposition of Tin Alloys From Alkaline StannateBaths, by Monk & Ellingham, Trans. of the Faraday Soc., No. 174, vol.31, part 10, October 1935, pp. 1462- 1466.

Handbook of Chemistry and Physics, 32nd ed., The Chemical RubberPublishing Oo., 1950, pp. 546-547.

JOHN H. MACK, Primary 'Examiner R. L. ANDREWS, Assistant Examiner

